The present invention relates generally to operational amplifiers and more particularly to a circuit for increasing the voltage gain of operational amplifiers.
A commonly used operational amplifier has an input stage, a voltage gain stage and a current gain stage. The input stage provides a voltage input to the voltage gain stage, which normally consists of one or more serially connected voltage amplifiers. A well known circuit of this type has a voltage gain stage consisting of two amplifier circuits with each amplifier circuit consisting of an emitter follower transistor driving a common emitter output transistor. This arrangement provides a voltage gain stage which has a high output impedance and a relatively high voltage gain. The output of the last amplifier in the voltage gain stage is usually connected to a current gain stage (often comprising emitter follower transistors) which provides for isolation of the voltage gain stage from output load current variations. Thus, the current gain stage, which has a voltage gain of approximately 1, acts as a buffer for the output of the operational amplifier.
While amplifiers of this type are characterized by a relatively high voltage gain, certain applications call for a voltage gain which is higher than that readily available from a single, commercially produced operational amplifier. Thus, there exists the need for a circuit or technique which provides for increasing the voltage gain of an operational amplifier of the type just described. In the case of a bipolar amplifier of this type, the present invention increases the gain by providing a feedback path for current to flow from an isolated current source to the voltage gain stage. This current reduces the current flowing in the common emitter output transistor of one of the amplifier circuits. This results in an effective increase in the output impedance of the common-emitter circuit, accompanied by a corresponding increase in circuit gain. In the particular embodiment of the invention described below, this is accomplished by the addition of a single component to a known operational amplifier circuit.
Accordingly, an object of the present invention is to provide an improved operational amplifier.
Another object of the present invention is to provide means for increasing the voltage gain of an operational amplifier without adversely affecting other amplifier performance characteristics.
Still another object of the present invention is to provide means for increasing the voltage gain of an operational amplifier by the addition of a minimal number of components.
These and other objects are attained in an operational amplifier which comprises an input stage, a voltage gain stage, a buffer stage and a feedback connection from the buffer stage to the voltage gain stage. The input stage transmits an input voltage signal to the input of the voltage gain stage. This stage has at least one amplifying circuit of the common-emitter type which includes a transistor having a collector connected to a current source, an emitter connected by an impedance to a reference voltage and a base connected to a control voltage responsive to variations in the input voltage. The collector is also connected to the input of a buffer stage having an output which is isolated from, but which tracks the voltage present on the collector. The feedback connection from the output of the buffer stage to the emitter of the amplifying circuit provides a path for current which varies in proportion to output voltage variations and which acts to reduce the collector current flowing in the amplifying device for a given instantaneous input voltage level, effectively increasing the output impedance and gain of the common emitter circuit. The feedback path includes an amplifying device to control the flow of feedback current. This device is preferably a transistor which is matched in characteristics to the transistor in the common-emitter circuit. The base of the matching transistor is connected to a voltage source which is substantially equivalent to the voltage supplied to the base of the amplifer circuit. The amplifier circuit can alternatively be implemented as a common-source circuit when field effect transistors are used. The operation of this circuit is as described above, with the drain, source and gate connections corresponding to the collector, emitter and base connections, respectively.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.